Temperature measuring system

ABSTRACT

For temperature measurement at a number of measuring points on, for example, current-carrying bars (2) in a high-voltage switchgear, a data transmitter (1) is used which is arranged near each measuring point and at the same potential as such point. The transmitter is controlled by a temperature sensor (8) and comprises components (3, 4) for capturing drive energy from the electric or the magnetic field around the bar. The temperature sensor (8) may consist of a quartz crystal, the resonance frequency of which has a specified temperature dependence. Measured data is transmitted by means of light (e.g. infrared) in the air to a data receiver (10) at ground potential. The data receiver (10) is able to receive measured data from a plurality of data transmitters.

TECHNICAL FIELD

The present invention relates to a device for measurement of temperatureon current conductors at high potential which is defined in more detailin the preamble to claim 1. By high potential are meant here voltagelevels from 220 V up to the highest system voltages occurring in powertransmission systems.

BACKGROUND ART

Within the electric power technique it is of great interest to know thetemperature of current carrying apparatus and busbars. This can be usedfor controlling the magnitude of the load (e.g. in case of temporaryoverload) and for monitoring undesired power development in busbarjoints, contacts and cable connections, whereby, for example, increasedresistances caused by corrosion may be detected.

What makes the above measuring problem especially difficult is that themeasuring points are positioned at high potential in connection withlarge current. The measuring system must handle high electric andmagnetic fields and must not increase the risk of creeping currents orflashover. Preferably, it should be designed such that no new type testsneed to be made to verify the safety.

In the publication Siemens-Energietechnik 6 (1984) No. 6, pp. 277-279,Jez et al: "Lichtleiter-Trennverstarker, ein Gerat zum Messen auf hohemPotential", a measuring device intended, inter alia, for temperaturemeasurement in high-voltage plants is described. A considerable drawbackin this device is that the current supply to the measured datatransmitter takes place with the aid of an accumulator placed at highpotential near the transmitter, which results in the high-voltage partof the equipment becoming bulky and maintenance-demanding. Astemperature sensors in this equipment there are used thermocouples,which results in a weak analogue signal which requires amplification andconversion. For transmission of measured information from thetransmitter at high potential to the receiver at ground potential, alight guide with optical fibres is used. This may in certain casesentail an increased risk of creeping currents and flashover.

The above-mentioned drawbacks can only be partly avoided by a device fortemperature measurement described in patent specification EP-A-0263233.This device comprises a temperature sensor in the form of a quartzcrystal. The crystal constitutes the frequency-determining element in anoscillator circuit, the output signal of which is converted into asequence of pulses which are sent out as light to a detector. Thetemperature of the sensor is obtained by measuring the period of thereceived signal. The current supply of the sensor unit is performed,also in this device, with the aid of a battery or from an evaluationunit via a cable. Such a solution is therefore hardly applicable totemperature measurements at high potential. In addition, the device isrelatively sensitive to disturbances from ambient light, since eachperiod of the signal on which the measurement is based contains only onepulse. The evaluation unit of the device is intended to receive datafrom only one transmitter, which means that in the case of applicationswhere temperature measurements are to be carried out at a plurality ofmeasuring points, measuring equipment designed in this way will becomerelatively costly.

From patent specification DE-B-1904853 it is also known to use, inconnection with temperature measurement in oil-filled transformers, aquartz crystal serving as a temperature sensor for control of anoscillator. The transmission of measured information to ground potentialis in this case performed with the aid of ultrasonics. Neither in thisdevice is it possible for one and the same receiver to receive anddistinguish measured data from several transmitters.

SUMMARY OF THE INVENTION

The present invention aims to provide a relatively simple andpractically maintenance free temperature measurement device for theabove-mentioned purposes, which does not suffer from the above-mentioneddisadvantages of prior art designs. The device is to be so small involume that it may be used, for example, in existing cubicle-enclosedmedium voltage switchgear. This is obtained according to the inventionby a device with the characteristic features described in the appendedclaims.

A measuring device according to the invention may comprise a pluralityof small electronics units which are placed at the measuring points sothat they are at the same potential as the measuring points from anelectrical point of view. The units are adapted to obtain their requireddrive energy from the magnetic field at the measuring point, measure thetemperature, convert measured data into a message and transmit thisthrough the air to a receiver which is positioned at ground potential.The message is built up such that a receiver can receive measured datafrom several transmitters.

A transmitter of measured data according to the invention is adapted totransmit, at the same time, both a digital identity code of its own andanalog measured information to the data receiver by modulating theoutput signal of a transmitter element in such a way that this outputsignal is in the form of short pulse showers which are repeated with arelatively long period, which is determined both by the identity code ofthe data transmitter and by the measured value. The measured value isthereby represented by a stepless deviation from a basic period which isselected for each data transmitter and which, in turn, represents theidentity code of the respective data transmitter. By transmitting themeasured information in the form of pulse showers, more reliable andsensitive receivers can be constructed than if the information wouldhave consisted only of one pulse per period. The reason for this is thatthe receiver can be allowed to be selective for the frequency occurringin the pulse shower, thus sorting out unwarranted signals, for examplefrom ambient light.

The proposed solution to allow the transmitters to transmit over theirown identity code, such that the same receiver can receive anddistinguish data from several transmitters, is advantageous in that itrequires hardly any extra hardware in the transmitter unit compared witha system without transmitter identification. In an application where thenumber of sensor units is large, it is important to keep the degree ofcomplexity in the sensor unit at a low level, and rather spend a littlemore on the receiver.

In a particularly suitable embodiment of the invention, theabove-mentioned electronics unit comprises an oscillator circuit, thefrequency of which is changed with the temperature. As temperaturesensor there is used directly a crystal (quartz resonator), theresonance frequency of which has a specified temperature dependence.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained in greater detail by describing anembodiment with reference to the accompanying drawing, wherein

FIG. 1 is a diagram showing the principle of a measuring deviceaccording to the invention,

FIG. 2 schematically shows a transmitter of measured data attached to acurrent-carrying bar, and

FIG. 3 shows examples of pulse trains emitted from three transmitterswhich are placed on different phase conductors in a three-phase system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The data transmitter shown in FIG. 2 is fixed, for example by a bindingstrap, to a current-carrying bar 2 in, for example, a switchgear cubiclefor medium voltage. (By medium volt, age is meant here the voltage range1-40 kV.) The transmitter 1 has a size of, for example, 10×40×40 mm andis designed with rounded corners for avoiding electric fieldconcentrations. The transmitter is provided with an energy supply unitcomprising a coil 3 with an iron core 4 (FIG. 1). The magnetic field Bgenerated by the current in the bar 2 induces in the coil 3 a voltagewhich is rectified in a rectifier 5. Energy is stored in a capacitor 6to continuously drive an electronics unit 7, and to transmit messages.The current in the bar 2 may vary from, for example, 50 A (operatingcurrent) to 40 kA (short-circuit current), which entails greatvariations in the induced voltage in the coil 3. However, because of theiron core 4, which becomes saturated at the higher currents, a certainstability of the energy from the coil is obtained. At too low magneticfields (=low currents), the temperature measurement function is allowedto cease.

As temperature sensor there is used a quartz crystal 8, the resonancefrequency of which has a specified temperature dependence. A suitablecrystal for this purpose is made by the Swiss firm Micro Crystal,CH-2540 Grenchen. This crystal has a resonance frequency of 262144 Hz at25° C., and the frequency variation is linear with about 7 Hz/1° C. Thecrystal 8 is part of a so-called Pierce oscillator in the electronicsunit 7. The oscillator signal from the temperature measurement isfrequency-divided in several stages, and is used to create a pulse trainconsisting of showers of pulses (e.g. 16 cycles) with a suitablefrequency (e.g. 33 kHz) which are transmitted at intervals which may beselected in the transmitter (e.g. 1 shower/8-16 s, which is selectedeither at the production or the commissioning stage). To enable areceiver, which receives measured data from several differenttransmitters, to identify the different pulse trains, different lengthsof the intervals between the showers in the different transmitters maybe selected, as is clear from FIG. 3. The intervals t_(R), t_(S) andt_(T) between the pulse showers P in the different phases R, S, T may,for example, be t_(R) =8 s, t_(S) =9 s and t_(T) =10 s. Because theintervals have different lengths, the advantage is also obtained thatpulse showers from different transmitters more seldom hit the receiverat the same time.

When the temperature of, for example, the bar 2 in phase varies, theintervals between the pulse showers from the data transmitter of the barwill be changed by an amount Δt from the value t_(R) exactly set at acertain temperature. This change Δt may amount to a few milliseconds andconstitutes a measure of the temperature change.

The pulse train resulting from the measured data conversion controls alight emitting diode (LED) 9 which emits light (visible or infrared). Toobtain a greater degree of scattering of the emitted light, twotransmitter diodes may be used, which are directed in differentdirections.

The transmission of measured information to ground potential takes placethrough the air, that is, without the use of light guides. Theinformation is captured by a data receiver 10 which comprises aphotodetector 11, which converts the light signal into an electricsignal which is processed for presentation in a suitable way.

Transmitting measured information in the form of short pulse showerswith relatively long intervals has the advantage, compared with atransmission with a continuous pulse train, that one and the samereceiver can receive and distinguish measured data from severaldifferent transmitters. In addition, the energy requirement is reduced,and the service life of the LED increases.

The transmission distance between the transmitter and the receiver mayvary between about 0.1 m in medium voltage switchgear and about 8 m inan 800 kV switchgear.

The invention is not limited to the embodiment shown and described butseveral modifications are possible within the scope of the claims. Forexample, instead of transmitting measured data by light, it is possibleto use ultrasonics for the transmission. The proposed device is alsosuitable for temperature measurements in oil-filled transformers, themeasured information then being transmitted through the transformer oil.

I claim:
 1. An apparatus for measuring temperature at a plurality ofmeasuring points on conductors operative at a high potential to carryelectrical current and to produce an electric and magnetic field whenoperative comprising:temperature sensing means at each measuring pointfor sensing the temperature of the conductor and producing a data signalfor each said measuring point; transmitting means responsivelyassociated with each measuring point and responsive to the data signalbeing located proximate thereto and at substantially the same potentialthereof for producing a corresponding output signal being modulated toindicate uniquely the identity of said measuring point and to indicatethe temperature for each such measuring point; each of said transmittingmeans operative when energized for modulating its corresponding outputsignal so that such output signal is in the form of pulse burstsrepeated with a selected basic period of time duration different thaneach of other basic periods so that each such basic period correspondsuniquely to the associated measuring point, said time duration of eachbasic period representing each measuring point and being variable up toa maximum deviation, said basic period being long relative to theduration of said pulse bursts, and said deviation of the output signalfrom the basic period representing the temperature of the measuringpoint and the time differences between the basic periods beingrelatively larger than said maximum deviation of said period; and anenergy unit responsive to at least one of the electric and magneticfield produced by the conductors for obtaining energy therefrom andoperatively coupled to the temperature sensing means and the transmittermeans for energizing the same.
 2. The device according to claim 1further comprising a receiver at ground potential for receiving theoutput signal.
 3. The device according to claim 2 wherein thetransmission of the output signal between the transmitter and thereceiver takes place in at least one of air, a gaseous medium and aliquid medium.
 4. The device according to claim 1 wherein thetemperature sensor comprises a quartz crystal and an oscillator circuitcoupled thereto, said quartz crystal having a resonance frequency with aspecified temperature dependence.
 5. The device according to claim 1wherein the transmitting means comprises a converter of the data signaland including a light emitting diode responsive to the output signal foremitting modulated light in the infrared region.
 6. The device accordingto claim 1 wherein the transmitter means comprises an ultrasonictransmitter.